1. Field of the Invention
The present invention relates generally to imaging apparatus and methods, and more particularly to an apparatus and method for producing high definition grayscale images using context sensitive pixel modulation.
2. Brief Description of the Field
Imaging systems have improved dramatically over the past several years, requiring increased emphasis on designing system components which contribute in producing high quality final output images at increased overall savings to the consumer. The drive for high quality imaging devices, i.e., printers and monitors, has mandated increased efficiency in allocating expensive electronic components, i.e., memory, while providing better resolution and greater numbers of colors or gray levels.
In traditional imaging systems, a code-word is used to describe the manner in which a given area is to be imaged. This code-word is normally a multi-bit data pixel which can, for example, represent a given gray level. In most prior art systems, a modulator takes this multi-bit data pixel and attempts to create a gray level over the area addressed by that pixel by writing "black" in a small imaged area in a field of "white". The human eye perceives the average of this area as a gray level. An inherent problem with this type of system, however, is that certain imaging devices, particularly laser scanning electrophotographic print engines, have more addressable points than their resolution supports. In other words, data pixels which do not overlap in the system's pixel map will overlap in the imaging device. A direct result of this is that writing a gray level next to a black, gray or white area all produce different final output image results.
One method for achieving better resolution in the final output image is to use pulse width modulation to image the output pixels. Briefly, pulse width modulation is a technique for subdividing each output pixel into a much smaller unit (in the above example, the unit of time that the laser is writing "black"). As an example of the benefits of pulse width modulation, let us assume that we have an imaging area comprised of a 4-pixel-by-4-pixel cell. Given the parameters of this cell, using traditional modulation we can achieve 17 gray levels (0/16, 1/16, 2/16, . . . , 16/16). A 5-pixel-by-5-pixel cell would enable us to achieve 26 gray levels, and so on. The main problem is that we need substantially more than 30 levels of gray in order to produce a final output image with good resolution without contouring. However, using the traditional method of imaging, the imaging area becomes too large when trying to use more than a 4-pixel-by-4-pixel imaging area.
We can overcome this limitation and squeeze more gray levels out of the 4.times.4 area by turning on partial pixels. That is, by dividing an output pixel into smaller units, we can achieve a substantially greater number of gray levels using the same imaging area. For instance, we could now have 0.6/16 as one achievable gray level (as opposed to the simple integer combinations described above).
However, a consequence of using pulse width modulation to image gray levels is that the position of dot growth can become very important. The "dot" produced by the laser can usually grow from the center, left, right, or in from the edges. Depending on the neighboring pixels surrounding the current pixel (the output pixel being currently imaged), differing results in gray level imaging can occur. Thus, there arise many situations in which one wants to add gray to the specific right or left of a given pixel to increase the resolution of the final output image and enhance the final overall output image quality.
Traditionally, shifting the portion or direction of pixel growth is done by relating a code word to the partial pixel being imaged. For example, 0001 could be used to designate a small pixel growth from the right and 1000 could be used to designate the same pixel growth, but from the left. Thus, when it is desired to shift the pixel growth toward a given imaged area, a code word would be associated with the partial pixel to be imaged in a manner that would enable this shifting. Unfortunately, this requires more frame store space in which to store the additional codewords to indicate pixel growth direction. This, consequently, greatly increases the cost of the imaging device.